1. Field of the Invention
The present invention relates to a clad material for a battery negative electrode lead material, a method of manufacturing a clad material for a battery negative electrode lead material and a battery including the clad material for a battery negative electrode lead material.
2. Description of the Background Art
In general, there is known a technique of employing Ni (nickel) having high corrosion resistance as a battery negative electrode lead material for connecting a negative electrode collector and a battery negative electrode with each other. However, Ni has high electrical resistance, and hence power loss is disadvantageously increased following higher output or downsizing of a battery. On that account, there is known a technique of employing Cu (copper) having low electrical resistance as the core of a battery negative electrode lead material, in order to improve electrical conductivity.
For example, Japanese Patent Laying-Open No. 2003-203622 discloses an in-battery lead material connecting a negative electrode and a container of stainless constituting a negative electrode terminal with each other. This in-battery lead material consists of a clad material of a three-layer structure prepared by integrally bonding a core (may hereinafter be referred to as a “Cu layer”) constituted of pure Cu or a Cu alloy mainly composed of Cu and a pair of connecting materials (may hereinafter be referred to as “Ni layers”) constituted of pure Ni or an Ni alloy mainly composed of Ni and arranged on both sides of the core to each other.
Japanese Patent Laying-Open No. 2003-203622 discloses that the clad material of the three-layer structure prepared by bonding the Ni layer, the Cu layer and the Ni layer to each other for the in-battery lead material is heat-treated under a condition of holding the same in the temperature range of 500° C. to 600° C. for one hour. If batch heat treatment is performed on the clad material under a condition of holding the same at a temperature of 600° C. for one hour, for example, dispersions in thicknesses of diffusion layers formed on bonded regions between the Cu layer and the Ni layers are easily increased. When such diffusion layers remarkably dispersed in thicknesses are formed, the thickness of the Cu layer may be partially reduced, to result in high electrical loss in the in-battery lead material. If the batch heat treatment is performed on the clad material under a condition of holding the same at a temperature of 500° C. for one hour, for example, welding strength between the in-battery lead material and the container (the negative electrode terminal) may be hard to ensure when the former is resistance-welded to the latter.